Vapor compression systems circulate refrigerant in a closed loop to transfer heat from one external medium to another external medium. Vapor compression systems are used in air-conditioning, heat pump, and refrigeration systems. FIG. 1 depicts a conventional vapor compression heat transfer system 100 that operates though the compression and expansion of a refrigerant fluid. The system 100 transfers heat in one direction from a first external medium 150, through a closed-loop, to a second external medium 160. Fluids include liquid and/or gas phases. Thus, if the first external medium 150 was the indoor air contained by a structure, and the second external medium 160 was the air outside of the structure, the system 100 would cool the indoor air during operation.
A compressor 110 or other compression device reduces the volume of the refrigerant, thus creating a pressure difference that circulates the refrigerant through the loop. The compressor 110 may reduce the volume of the refrigerant mechanically or thermally. The compressed refrigerant is then passed through a condenser 120 or heat exchanger, which increases the surface area between the refrigerant and the second external medium 160. As heat transfers to the second external medium 160 from the refrigerant, the refrigerant contracts in volume.
When heat transfers to the compressed refrigerant from the first external medium 150, the compressed refrigerant expands in volume. This expansion is often facilitated with a metering device 130 including an expansion device and a heat exchanger or evaporator 140. The evaporator 140 increases the surface area between the refrigerant and the first external medium 150, thus increasing the heat transfer between the refrigerant and the first external medium 150. The transfer of heat into the refrigerant from the evaporator 140 causes at least a portion of the expanded refrigerant to undergo a phase change from liquid to gas. Thus, air contacting the surface of the evaporator 140 undergoes a reduction in temperature. The heated refrigerant is then passed back to the compressor 110 and the condenser 120, where at least a portion of the heated refrigerant undergoes a phase change from gas to liquid when heat transfers to the second external medium 160. Thus, air contacting the surface of the condenser 120 undergoes an increase in temperature.
The closed-loop heat transfer system 100 may include other components, such as a compressor discharge line 115 joining the compressor 110 and the condenser 120. The outlet of the condenser 120 may be coupled to a condenser discharge line 125, and may connect to receivers for the storage of fluctuating levels of liquid, filters and/or desiccants for the removal of contaminants, and the like (not shown). The condenser discharge line 125 may circulate the refrigerant to one or more metering devices 130.
The metering device 130 may include one or more expansion devices. The metering device 130 includes the ability to alter the rate of refrigerant flow through the device. An expansion device may be any device capable of expanding, or metering a pressure drop in the refrigerant at a rate compatible with the desired operation of the system 100. Thus, the metering device 130 alters the rate of refrigerant flow, and when including an expansion device, also includes the ability to meter a pressure drop in the refrigerant.
The metering device 130 may provide a static orifice or may adjust during operation of the system 100. The static orifice may be in the form of an adjustable valve that is set and not changed during operation of the system 100. Orifices that adjust during operation may have mechanical or electrical control. For example, mechanical control during operation could be provided by a bi-metal spring that adjusts tension or by a fluid that adjusts the pressure exerted against a diaphragm in response to changes in pressure or temperature. Similarly, electrical control during operation could be provided by a servo motor that changes the orifice in response to the electrical signal from a thermocouple.
Useful metering devices having the ability to expand the refrigerant (meter a pressure drop in the refrigerant) include thermal expansion valves, capillary tubes, fixed and adjustable nozzles, fixed and adjustable orifices, electric expansion valves, automatic expansion valves, manual expansion valves, and the like. Examples of thermal expansion valves include the Sporlan EBSVE-8-GA (one-directional) and the Sporlan RZE-6-GA (bi-directional), as available from Parker Hannifin, Cleveland, Ohio. Examples of capillary tubes include the Sporlan Style F and the Supco BC 1-5, as available from Supco, Allenwood, N.J. Examples of electric expansion valves include the Parker SER 6 and 11, as available from Parker Hannifin, Cleveland, Ohio. Other metering devices may be used.
The refrigerant exiting the expansion portion of the metering device 130 passes through an expanded refrigerant transfer system 135, which may include one or more refrigerant directors 136, before passing to the evaporator 140. The expanded refrigerant transfer system 135 may be incorporated with the metering device 130, such as when the metering device 130 is located close to or integrated with the evaporator 140. Thus, the expansion portion of the metering device 130 may be connected to one or more evaporators by the expanded refrigerant transfer system 135, which may be a single tube or include multiple components. The metering device 130 and the expanded refrigerant transfer system 135 may have fewer or additional components, such as described in U.S. Pat. Nos. 6,751,970 and 6,857,281, for example.
One or more refrigerant directors 136 may be incorporated with the metering device 130, the expanded refrigerant transfer system 135, and/or the evaporator 140. Thus, the functions of the metering device 130 may be split between one or more expansion device and one or more refrigerant directors and may be present, separate from, or integrated with the expanded refrigerant transfer system 135 and/or the evaporator 140. Useful refrigerant directors include tubes, nozzles, fixed and adjustable orifices, distributors, a series of distributor tubes, direction-altering valves, and the like.
The evaporator 140 receives the expanded refrigerant in a substantially liquid state with a small vapor fraction and provides for the transfer of heat to the expanded refrigerant from the first external medium 150 residing outside of the closed-loop heat transfer system 100. Thus, the evaporator or heat exchanger 140 facilitates in the movement of heat from one source, such as ambient temperature air, to a second source, such as the expanded refrigerant. Suitable heat exchangers may take many forms, including copper tubing, plate and frame, shell and tube, cold wall, and the like. Many conventional systems are designed and operated, at least theoretically, to completely convert the liquid portion of the refrigerant to vaporized refrigerant within the evaporator 140. In addition to the heat transfer converting liquid refrigerant to a vapor phase, the vaporized refrigerant may become superheated, thus having a temperature in excess of the refrigerant's boiling temperature and/or increasing the pressure of the refrigerant. The refrigerant exits the evaporator 140 through an evaporator discharge line 145 and returns to the compressor 110.
In conventional vapor compression systems, the expanded refrigerant enters the evaporator 140 at a temperature that is significantly colder than the temperature of the air surrounding the evaporator. As heat transfers to the refrigerant from the evaporator 140, the refrigerant temperature increases in the later or downstream portion of the evaporator 140 to a temperature above that of the air surrounding the evaporator 140. This rather significant temperature difference between the initial or inlet portion of the evaporator 140 and the later or outlet portion of the evaporator 140 may lead to oil retention and frosting problems at the inlet portion.
FIG. 2A and FIG. 2B depict a conventional heat pump system 200 having the capability to transfer heat in two directions. Thus, while system 100 can transfers heat from the first external medium 150 to the second external medium 160, the heat pump system 200 can transfer heat from a first external medium 250 to a second external medium 260 (FIG. 2A) or can transfer heat from the second external medium 260 to the first external medium 250 (FIG. 2B). In this manner, the system 200 may be considered “reversible” in its ability to transfer heat.
In a conventional heat pump implementation, an inside heat exchanger 240 is placed within a conditioned space, while an outside heat exchanger 220 is placed outside of the conditioned space, generally outdoors. The conditioned space may be the interior of a home, vehicle, refrigerator, cooler, freezer, and the like.
In cooling mode, where the system is transferring heat from the conditioned space to the outdoors, the inside heat exchanger 240 is serving as the evaporator, while the outside heat exchanger 220 is serving at the condenser. In reversed, or heat pump mode, where the system is transferring heat from the outdoors to the conditioned space, the inside heat exchanger 240 is serving as the condenser, while the outside heat exchanger 220 is serving at the evaporator. Thus, regardless of operation mode, heat is always being transferred into the evaporator and away from the condenser.
Unlike the one-directional system 100, the bi-directional heat pump system 200 uses a flow reverser 280 and two metering devices 230, 233, which may pass refrigerant in either direction. As the compressor 210 passes refrigerant in one direction, the flow reverser 280 allows either the inside heat exchanger 240 or the outside heat exchanger 220 to feed an evaporator discharge line 245 that feeds the low pressure inlet side of the compressor 210. Thus, the flow reverser 280 switches the system between heating or cooling the first external medium 250. Examples of flow reversers include the Ranco V2 and V6 products, as available from Invensys, Portland House, Bressenden Place, London. Other flow reversers may be used.
At any one time, one of the metering devices is functioning to expand and/or meter a pressure drop in the refrigerant while the second metering device is back-flowing refrigerant and not functioning to expand the refrigerant. Thus, in FIG. 2A where heat is being removed from the first external medium 250 to cool the conditioned space, the metering device 230 is expanding the refrigerant, while the metering device 233 is back-flowing refrigerant. Similarly, in FIG. 2B where heat is being provided from the second external medium 260 to heat the first external medium 250 to the conditioned space, the metering device 233 is expanding the refrigerant while the metering device 230 is back-flowing refrigerant.
If either of the metering devices 230, 233 are not bi-directional, thus lacking the ability to back-flow the refrigerant and maintain the desired performance, one-directional metering devices may be used in combination with bypass loops 271, 272 including one-directional check valves 270, 273, as represented in FIG. 2C (cooling) and in FIG. 2D (heating). Thus, while one metering device expands the refrigerant, the second metering device is bypassed with a bypass loop and a check valve. The check valve prevents refrigerant from back-flowing through the associated one-directional metering device.
A disadvantage of conventional heat pumps is that because they serve two functions (heating and cooling the same conditioned space), they are not optimized for either. One way the heat pump system 200 represented in FIG. 2B provides heat at the inside heat exchanger 240 is by introducing a restriction to refrigerant flow in an expanded refrigerant transfer system 235. While such a restriction could be located anywhere in the expanded refrigerant transfer system 235 allowing for proper operation of the system, the restriction is often incorporated into one or more refrigerant directors 236. By making the refrigerant directors 236 smaller than optimal for cooling, refrigerant reaches a higher temperature and pressure in the inside heat exchanger 240 during heating as it is more difficult for the refrigerant to exit the inside heat exchanger 240. Thus, while the system 200 can provide heat to the indoor space, the cooling efficiency provided by the system is substantially reduced as the restriction also restricts refrigerant from entering the inside heat exchanger 240 during cooling.
In addition to the energy wasted from operating the compressor 210 at a higher pressure than would otherwise be needed for optimal cooling efficiency, as the compressor 210 works against the restriction when heating and when cooling, the operational lifetime of the compressor 210 is reduced in relation to a system where the compressor 210 works harder when heating, but not when cooling.
Although heat pumps are generally used to heat conditioned spaces in temperate climates, heat pumps may be used in colder regions, such as when only electricity is available and resistance coils are undesired. Colder regions are those where winter average low temperatures are about 0° C. and below. Much colder regions are those where winter average low temperatures are about −7° C. and below. As winter average low temperatures decrease from about 0° C., heat pump usage declines significantly. For example, in the much colder regions of the United States, such as the East North Central, West North Central, and Mountain regions, heat pump usage is less than 10% in newer single family homes, while averaging about 47% in the warmer South Atlantic, East South Central, and West South Central regions.
While heat pumps may be used in these colder regions, if the frost built up on the outside heat exchanger 220 during on-cycles of the compressor 210 (heating) does not substantially melt during off-cycles, defrost cycles may be necessary to remove the frost and restore heat transfer efficiency to the system 200. As the temperature of the outside heat exchanger 220 drops as heat is transferred to the inside heat exchanger 240, the ability of the outside heat exchanger 220 to extract heat from the outdoors, while maintaining a surface temperature above 0° C. to prevent frosting, decreases with lower outdoor air temperatures.
Thus, in heating mode, where the outside heat exchanger 220 is functioning as an evaporator, frosting of the outside heat exchanger 220 can be a significant problem requiring frequent defrosting. Such frosting often is caused by expanded refrigerant in the initial portion of the outside heat exchanger 220 being at a temperature below the dew point of the outside air, which results in moisture condensation and freezing on the outside heat exchanger 220 during heating operation. Thus, as with an indoor evaporator used for cooling, the outside heat exchanger 220 of a heat pump system can freeze during heating. In fact, the problem can be more severe for the outside heat exchanger of a heat pump system as the system cannot significantly alter the humidity content of the outside air and the outdoor air temperature when heating is generally lower than the conditioned space air temperature when cooling.
As frost encloses a portion of the outside heat exchanger's surface during heating, the frosted surface insulates the coils of the outside heat exchanger 220 from direct contact with the outdoor air. Consequently, airflow over and/or through the outside heat exchanger 220 is reduced and the ability of the outside heat exchanger 220 to absorb heat from the outdoors (heating efficiency) decreases. Thus, the amount of heat that the heat pump system 200 can transfer from outdoors to the conditioned space decreases for the energy consumed (a reduction in heating efficiency) and the rate at which the system 200 can transfer heat from outdoors to the conditioned space also decreases. This reduction in the rate of heat transfer results in a decrease in the temperature of the heated air that is provided to the conditioned space.
Conventional heat pump systems may passively defrost by turning off the compressor 210 or may actively defrost by applying heat to the outside heat exchanger 220 during defrost cycles. Whether one or both methods are used, defrosting requires a larger vapor compression system than would be required if the system did not have to suspend the desired direction of heat transfer to defrost.
As the compressor 210 is off during passive defrosting, the rate at which the system 200 can heat the conditioned space is reduced. Passive defrost cycles may be controlled by a simple timing mechanism, such as when the compressor 210 remains on for 30% of a selected time period, regardless of the amount of heat desired for the conditioned space. Passive defrost cycles also may be controlled by electronic circuits that monitor the performance of the outside heat exchanger 220 and attempt to maximize operation of the compressor 210 in relation to the efficiency lost due to frosting of the outside heat exchanger 220.
For active defrosting, heat is generally transferred from the conditioned space to the outside heat exchanger 220 by transferring heat that the system 200 previously transferred from the outdoors to the conditioned back to the outside heat exchanger 220. Thus, the heat pump system is operated in cooling mode even though the conditioned space requires heating, when actively defrosting the outside heat exchanger 220 and consumes energy to move heat back to where it started, outdoors. Additionally, as heated air from the conditioned space is blown across the inside heat exchanger 240 during active defrosting to prevent icing of the inside heat exchanger 240, supplemental heat may be provided by inductance coils or other means to prevent the system from providing cold air to the conditioned space. Thus, a conventional heat pump system requiring frequent defrosting often operates as a forced-air electric induction heater, which must heat the outside heat exchanger 220 in addition to the conditioned space. This results in any theoretical energy efficiency gain obtained from the transfer of heat from the outdoors to the conditioned space to be lost.
Accordingly, there is an ongoing need for heat pump systems having improved efficiency when cooling and heating. It also would be desirable for heat pump systems to have an enhanced resistance to outside heat exchanger frosting during heating, especially in colder regions. The disclosed systems, methods, and devices overcome at least one of the disadvantages associated with conventional heat pump systems.